A so-called white Light Emitting Diode (LED) is a solid state device that converts electrical energy into light. The white LED is actually an assembly that includes a blue LED die or blue LED dice. Within a blue LED die, light is emitted from an active layer of semiconductor materials sandwiched between oppositely doped layers when an adequate voltage is applied across the doped layers. In one example of a blue LED die, an N-type GaN layer is disposed over a substrate, an InGaN/GaN Multiple Quantum Well (MQW) active layer is disposed over the N-type layer, and a P-type GaN layer is disposed over the active layer. A first electrode makes electrical contact with the N-type layer. A second electrode makes electrical contact with the P-type layer. When a current is made to flow from the second electrode, through the LED structure, to the first electrode, then light is emitted from the active layer. The light emitted may be said to be “blue” light because the spectrum of the light emitted has a peak intensity at or close to that of blue light. To make the light emitted from the overall LED assembly “white”, a layer of a material referred to generally as “phosphor” is disposed over the blue LED die. This phosphor actually involves phosphor particles suspended in a binding material such as silicone. Blue light emitted from the blue LED die is absorbed by these phosphor particles, and the phosphor particles in turn re-emit light. The light re-emitted is, however, of different wavelengths. The spectrum of all the light emitted from the overall white LED assembly is therefore broader than the spectrum of the blue light emitted from the blue LED itself. This broad spectrum roughly approximates the spectrum of white light. Such an assembly is therefore said to be a “white LED”.
A white LED may involve one blue LED die, or may involve a plurality of blue LED dice that are series-connected in a string. The blue LED die or dice are mounted to a printed circuit board. A first terminal of the printed circuit board is electrically connected to a first end of the string of blue LED dice. The first end of the string is the cathode electrode of the last LED of the string. A second terminal of the printed circuit board is connected to a second end of the string of blue LED dice. The second end of the string is the anode electrode of the first LED of the string. An amount of phosphor is disposed over the LED dice to cover the LED dice but so as not to cover the substrate terminals. The substrate terminals are available for external circuitry to connect to the white LED assembly.